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SSL_CTX_need_tmp_rsa.3
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The callback is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected. .PP \&\fISSL_CTX_set_tmp_rsa()\fR sets the temporary/ephemeral \s-1RSA\s0 key to be used to be \&\fBrsa\fR. The key is inherited by all \s-1SSL\s0 objects newly created from \fBctx\fR with <\fISSL_new\fR\|(3)|\fISSL_new\fR\|(3)>. Already created \s-1SSL\s0 objects are not affected. .PP \&\fISSL_CTX_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key with a keysize larger than 512 bits is installed. .PP \&\fISSL_set_tmp_rsa_callback()\fR sets the callback only for \fBssl\fR. .PP \&\fISSL_set_tmp_rsa()\fR sets the key only for \fBssl\fR. .PP \&\fISSL_need_tmp_rsa()\fR returns 1, if a temporary/ephemeral \s-1RSA\s0 key is needed, for RSA-based strength-limited 'exportable' ciphersuites because a \s-1RSA\s0 key with a keysize larger than 512 bits is installed. .PP These functions apply to \s-1SSL/TLS\s0 servers only. .SH "NOTES" .IX Header "NOTES" When using a cipher with \s-1RSA\s0 authentication, an ephemeral \s-1RSA\s0 key exchange can take place. In this case the session data are negotiated using the ephemeral/temporary \s-1RSA\s0 key and the \s-1RSA\s0 key supplied and certified by the certificate chain is only used for signing. .PP Under previous export restrictions, ciphers with \s-1RSA\s0 keys shorter (512 bits) than the usual key length of 1024 bits were created. To use these ciphers with \s-1RSA\s0 keys of usual length, an ephemeral key exchange must be performed, as the normal (certified) key cannot be directly used. .PP Using ephemeral \s-1RSA\s0 key exchange yields forward secrecy, as the connection can only be decrypted, when the \s-1RSA\s0 key is known. By generating a temporary \&\s-1RSA\s0 key inside the server application that is lost when the application is left, it becomes impossible for an attacker to decrypt past sessions, even if he gets hold of the normal (certified) \s-1RSA\s0 key, as this key was used for signing only. The downside is that creating a \s-1RSA\s0 key is computationally expensive. .PP Additionally, the use of ephemeral \s-1RSA\s0 key exchange is only allowed in the \s-1TLS\s0 standard, when the \s-1RSA\s0 key can be used for signing only, that is for export ciphers. Using ephemeral \s-1RSA\s0 key exchange for other purposes violates the standard and can break interoperability with clients. It is therefore strongly recommended to not use ephemeral \s-1RSA\s0 key exchange and use \s-1DHE\s0 (Ephemeral Diffie\-Hellman) key exchange instead in order to achieve forward secrecy (see \&\fISSL_CTX_set_tmp_dh_callback\fR\|(3)). .PP An application may either directly specify the key or can supply the key via a callback function. The callback approach has the advantage, that the callback may generate the key only in case it is actually needed. As the generation of a \&\s-1RSA\s0 key is however costly, it will lead to a significant delay in the handshake procedure. Another advantage of the callback function is that it can supply keys of different size while the explicit setting of the key is only useful for key size of 512 bits to satisfy the export restricted ciphers and does give away key length if a longer key would be allowed. .PP The \fBtmp_rsa_callback\fR is called with the \fBkeylength\fR needed and the \fBis_export\fR information. The \fBis_export\fR flag is set, when the ephemeral \s-1RSA\s0 key exchange is performed with an export cipher. .SH "EXAMPLES" .IX Header "EXAMPLES" Generate temporary \s-1RSA\s0 keys to prepare ephemeral \s-1RSA\s0 key exchange. As the generation of a \s-1RSA\s0 key costs a lot of computer time, they saved for later reuse. For demonstration purposes, two keys for 512 bits and 1024 bits respectively are generated. .PP .Vb 4 \& ... \& /* Set up ephemeral RSA stuff */ \& RSA *rsa_512 = NULL; \& RSA *rsa_1024 = NULL; .Ve .PP .Vb 3 \& rsa_512 = RSA_generate_key(512,RSA_F4,NULL,NULL); \& if (rsa_512 == NULL) \& evaluate_error_queue(); .Ve .PP .Vb 3 \& rsa_1024 = RSA_generate_key(1024,RSA_F4,NULL,NULL); \& if (rsa_1024 == NULL) \& evaluate_error_queue(); .Ve .PP .Vb 1 \& ... .Ve .PP .Vb 3 \& RSA *tmp_rsa_callback(SSL *s, int is_export, int keylength) \& { \& RSA *rsa_tmp=NULL; .Ve .PP .Vb 24 \& switch (keylength) { \& case 512: \& if (rsa_512) \& rsa_tmp = rsa_512; \& else { /* generate on the fly, should not happen in this example */ \& rsa_tmp = RSA_generate_key(keylength,RSA_F4,NULL,NULL); \& rsa_512 = rsa_tmp; /* Remember for later reuse */ \& } \& break; \& case 1024: \& if (rsa_1024) \& rsa_tmp=rsa_1024; \& else \& should_not_happen_in_this_example(); \& break; \& default: \& /* Generating a key on the fly is very costly, so use what is there */ \& if (rsa_1024) \& rsa_tmp=rsa_1024; \& else \& rsa_tmp=rsa_512; /* Use at least a shorter key */ \& } \& return(rsa_tmp); \& } .Ve .SH "RETURN VALUES" .IX Header "RETURN VALUES" \&\fISSL_CTX_set_tmp_rsa_callback()\fR and \fISSL_set_tmp_rsa_callback()\fR do not return diagnostic output. .PP \&\fISSL_CTX_set_tmp_rsa()\fR and \fISSL_set_tmp_rsa()\fR do return 1 on success and 0 on failure. Check the error queue to find out the reason of failure. .PP \&\fISSL_CTX_need_tmp_rsa()\fR and \fISSL_need_tmp_rsa()\fR return 1 if a temporary \&\s-1RSA\s0 key is needed and 0 otherwise. .SH "SEE ALSO" .IX Header "SEE ALSO" \&\fIssl\fR\|(3), \fISSL_CTX_set_cipher_list\fR\|(3), \&\fISSL_CTX_set_options\fR\|(3), \&\fISSL_CTX_set_tmp_dh_callback\fR\|(3), \&\fISSL_new\fR\|(3), \fIciphers\fR\|(1)